1. Field of the Invention
This invention relates, in general, to encapsulating compositions and, more specifically, to encapsulating compositions for electrical apparatus.
2. Description of the Prior Art
Electrical apparatus, such as transformers, generate considerable quantities of heat during their operation which must be adequately dissipated if the device is to operate reliably. Many different methods are used to remove this heat, including circulating air or coolant fluid around the electrical apparatus. One method used extensively with small transformers consists of encapsulating the transformer in a solid potting composition. This potting composition has higher thermal conductivity than air or oil and, as such, conducts considerable quantities of heat away from the transformer to the walls of the enclosure where it is carried off into the surrounding atmosphere.
A common type of potting composition, as shown in U.S. Pat. No. 2,941,905 issued to Hofmann and assigned to the assignee of the present application, includes an inert filler material, such as sand, which is mixed with a liquid synthetic resin to form an infusible mass around the electrical apparatus after curing of the resin. In addition, other types of inert filler materials, such as rounded gravel particles, have been added to the sand to reduce costs and improve the thermal conductivity of the potting composition. Several types of encapsulating compositions utilize resin-coated sand particles, or shell molding sand, in which each particle is covered by a partially cured coating of a thin film of resin. The resin coating, known as a "B" stage resin, is dry at ordinary room temperatures, but enters a fluid state when subjected to an elevated temperature and fuses to adjacent particles at the points of contact therebetween as it hardens or cures. Thus, in U.S. Pat. No. 3,161,843 to Hodges et al, the resin-coated sand is used to form the encapsulating composition with an insulating varnish added to completely fill the interstices between adjacent sand particles. Similarly, resin-coated sand has been mixed with large, rounded gravel particles to form the encapsulating composition, as disclosed in patent application Ser. No. 751,782, filed Dec. 16, 1976 in the names of Jaklic and Stephens, now U.S. Pat. No. 4,082,916, which is and assigned to the assignee of the present application.
Although the use of shell molding sand in encapsulating compositions simplifies manufacture and is considerably less expensive than the use of liquid synthetic resins and inert filler materials, several difficulties arise when it is used to form encapsulating compositions for electrical apparatus. During cure, the resin coating on each filler particle fuses to the resin coating on contiguous particles, thereby forming a bond only at the points of contact between the particles, as noted in U.S. Pat. Nos. 3,161,843 and 4,082,916 and U.S. Pat. No. 2,991,267 issued to Bean. Since the resin does not flow onto adjacent surfaces, such as the case walls, the uncoated filler particles or the electrical apparatus, adhesion to these surfaces is minimal, which thereby impedes heat transfer from the electrical apparatus to the encapsulating composition and from it to the enclosure. In addition, shrinkage during cure causes small gaps to appear between the encapsulating composition and adjacent portions of the case and electrical apparatus which, besides impeding heat transfer, results in higher noise levels. Furthermore, the case of large filler particles reduces the compressive strength and thermal stability of such potting compositions to undesirable levels.
Thus, it would be desirable to provide an encapsulating composition which has greater adhesion and compressive strength than similar prior art compositions. It would also be desirable to provide an encapsulating composition which has greater thermal conductivity and thermal stability properties than prior art compositions. It would also be desirable to provide an encapsulating composition, a portion of which contains resin-coated filler particles, in which the resin coating is made to flow between contiguous particles and also onto adjacent uncoated surfaces to form beads of resin therebetween.